When the plane hit Elia Zedeno's building on 9/11, the effect was not subtle. From the 73rd floor of Tower 1, she heard a booming explosion and felt the building actually lurch to the south, as if it might topple. It had never done that before, even in 1993 when a bomb exploded in the basement, trapping her in an elevator. This time, Zedeño grabbed her desk and held on, lifting her feet off the floor. Then she shouted, "What's happening?" You might expect that her next instinct was to flee. But she had the opposite reaction. "What I really wanted was for someone to scream back, 'Everything is O.K.! Don't worry. It's in your head.'"

She didn't know it at the time, but all around her, others were filled with the same reflexive incredulity. And the reaction was not unique to 9/11. Whether they're in shipwrecks, hurricanes, plane crashes or burning buildings, people in peril experience remarkably similar stages. And the first one--even in the face of clear and urgent danger--is almost always a period of intense disbelief.

Luckily, at least one of Zedeño's colleagues responded differently. "The answer I got was another co-worker screaming, 'Get out of the building!'" she remembers now. Almost four years later, she still thinks about that command. "My question is, What would I have done if the person had said nothing?"

Most of the people who died on 9/11 had no choice. They were above the impact zone of the planes and could not find a way out. But investigators are only now beginning to understand the actions and psychology of the thousands who had a chance to escape. The people who made it out of the World Trade Center, for example, waited an average of 6 min. before heading downstairs, according to a new National Institute of Standards and Technology (NIST) study drawn from interviews with nearly 900 survivors. But the range was enormous. Why did certain people leave immediately while others lingered for as long as half an hour? Some were helping co-workers. Others were disabled. And in Tower 2, many were following fatally flawed directions to stay put. But eventually everyone saw smoke, smelled jet fuel or heard someone giving the order to leave. Many called relatives. About 1,000 took the time to shut down their computers, according to NIST.

In other skyscraper fires, staying inside might have been exactly the right thing to do. In the case of the Twin Towers, at least 135 people who theoretically had access to open stairwells--and enough time to use them--never made it out, the report found.Since the early days of the atom bomb, scientists have been trying to understand how to move masses of people out of danger. Engineers have fashioned glowing exit signs, sprinklers and less flammable materials. Elaborate computer models can simulate the emptying of Miami or the Sears Tower, showing thousands of colored dots streaming for safety like a giant Ms. Pac-Man colony. But the most vexing problem endures. And it is not signage or architecture or traffic flow. It's us. Large groups of people facing death act in surprising ways. Most of us become incredibly docile. We are kinder to one another than normal. We panic only under certain rare conditions. Usually, we form groups and move slowly, as if sleepwalking in a nightmare.